1. Field of the Invention
The invention relates to computer keyboards, particularly keyboards performing n-key rollover, and more particularly to keyboards having n-key rollover without diodes at each key contact.
2. Description of the Related Art
Keyboards are used as the input device for a great many electronic devices. As an example, personal computers have a large keyboard, having 101 keys for example. This large number of keys provides various alphanumeric keys, function and control keys, cursor and editing keys and separate numeric keys. Because of the large number of keys, the keyboards are relatively complicated devices, having a large number of parts. As a result, it is desirable to reduce the cost and parts count of the keyboard.
Because of the large number of keys, it is standard to logically arrange the keys in a matrix and then scan the matrix. A number of lines are output or driven lines and are connected to one direction of the matrix, either rows or columns. A multi-bit data value is provided to these output lines, with one bit being different then the others. This bit is the active bit. The other direction of the matrix is the input, with the keys making connection between the rows and the columns. When a value is being driven on the output lines, the inputs are monitored. Any input having the same level as the active bit is considered closed. After the input lines are read, the active bit in the output value is changed and the inputs read again. This process is repeated continuously as the keyboard scanning function. Thus the state of each key in the keyboard in monitored. When a key is closed or made, as indicated by the particular input at a particular output value being at the active level, the circuitry provides the associated make key code to the computer, resulting in a key input to the computer. When the key is released, as indicated by the particular input at the same output value returning to the inactive level, the circuitry provides an associated break key code.
Typically keyboards come in two main types, two key rollover and n-key rollover. In two key rollover keyboards, the keyboard keeps track of the simultaneous depression of up to two different keys. This is relatively simple to do and so is very cost effective. However, there are many instances in current personal computers where three keys must be simultaneously depressed to perform special functions. Additionally, it is not uncommon for faster typists to be depressing several keys simultaneously in the desired sequence because of the high typing rate and the various mechanical travel distances. Therefore two key rollover keyboards cannot readily be used in personal computers. The problem can be partially alleviated by having certain special keys interpreted specially, but this increases cost of the keyboard.
Therefore n-key rollover keyboards are highly desirable. However, when just simple mechanical closures are used to form the key contacts, such as the various membrane dome mechanisms or other mechanical closure techniques as commonly utilized, a problem called a "phantom key" can result. If three of the four keys forming a rectangle in the key matrix are depressed, the fourth key is perceived to be closed. This occurs because a closed path is available through the three keys to simulate the closure of the fourth key. Obviously this is very undesirable. To resolve this problem it is necessary to install a diode in series with the contacts of each key. The diode prevents the reverse flow of current through the keys, so that the phantom key problem is resolved. Thus n-key rollover keyboards can be developed, but they have diodes at each key contact.
This requirement of the diodes has numerous undesirable side effects. First, the cost of the keyboard is directly increased because of the need to purchase the diodes. Second, assembly of the keyboard is more expensive because of the increased parts count. Third, the matrix organization requires the use of a double sided circuit board, which further increases direct and manufacturing costs.
Therefore it is clearly desirable to develop an n-key rollover keyboard without requiring the use of diodes at each key contact and having a simplified construction.